More and more user terminals, such as in particular personal computers (PCs) and personal digital assistants (PDAs), are based on generic platforms on which software is implemented. This trend relates in particular to audio/video readers.
However, the capabilities of terminals are often very different, both in terms of CPU (standing for “Central Processing Unit”) performance and in terms of available memory. It is therefore necessary to define techniques that tailor either the content to the terminal, or the terminal to the content.
Known solutions relating to the tailoring of the terminal to the content consist in performing a selective decoding if the terminal does not succeed in decoding all the data in real time. For example, during the receipt of an MPEG2 (standing for “Moving Picture Experts Group”) video stream, which conventionally comprises images of types I (decoded by themselves), P (decoded with the aid of earlier images) and B (decoded with the aid of earlier and later images), the overwhelmed terminal does not decode the B images (the latter not being used for the prediction of the subsequent images).
In transmission applications with no return path, by broadcasting or multicasting, this solution allows effective adaptation, although it leads to information losses that may prejudice the quality of reception.
By contrast, in point-to-point applications, it is possible to avoid this problem by tailoring the send bit rates to the terminals. To do this, according to a known technique, a prior step of negotiation is implemented between the target terminal and a client so that the performance of this terminal is communicated to the sender. The sender then chooses a content which may be decoded in real time by the terminal. This technique relies on the defining of a protocol making it possible to exchange information of this type, and on the defining of a measure of the performance of the terminal.
Among the methods disclosed pertaining to the tailoring of a sender to the capabilities of a receiver, Japanese document JP2000-270330 discloses a tailoring of send bit rates as a function of processing capabilities for terminals with decoders, by means of a prior notification to the sender of these capabilities by the receiver terminals.
It emerges from the disclosure of this document that a specific transmission protocol must be agreed between the sender and the receiver so as to achieve same, this having the drawback of requiring the implementation of a system that is coherent both as regards the senders and the receivers concerned. Moreover, such a procedure can benefit from a relatively wide application only in so far as it forms the subject of a recognized standardization.
Moreover, standardized techniques are commonly employed to tailor send streams to data circulation conditions within a point-to-point network. Thus, in the RTCP (standing for “Real-Time Control Protocol”), protocol the round trip time (RTT) for IP (standing for “Internet Protocol”) packets between a sender and a receiver is notified to the sender, which can take appropriate measures to modify the stream sent. This provision is particularly useful for reacting to congestion in the network. In the case of an algorithm of the “TCP-friendly” type, an error rate observed at the receiver (“loss event rate”, denoted “p”) is also communicated to the sender, which can thus tailor its send stream and generate protection to errors as a function of the error rate measured.
The use of the round trip time RTT has formed the subject of specific improvements aimed at taking account of the capacity of a buffer memory at the receiver level. Specifically, such a buffer memory prolongs the time to be taken into account when adjusting the bit rate of the send stream. Thus, document JP2001-257715 discloses a sender having an information processing section capable of calculating the RTT from an acknowledgement of receipt originating from the receiver. The processing section extracts a transmission bit rate corresponding to this RTT and to cues, appended to the acknowledgment of receipt, regarding the amount of storage of a buffer memory associated with the terminal. A bit rate control section then modifies the actual transmission bit rate on the basis of this extracted bit rate. This technique makes it possible to act on the transmission bit rate, so as to obtain good real-time reproduction and to avoid network congestions.
These methods, which make it possible to take account of the capacities of buffer memories for bit rate variations, require particular implementations at the levels of the senders and receivers, as well as specific communication protocols. Moreover, they leave intact the problem of tailoring to the processing capabilities of receivers.
A tempting solution for further improving the appropriateness of the send bit rate would consist in sending beforehand a notification informing the sender of the capabilities of the receiver, as described in document JP2000-270330, then in transmitting to the sender, during transmission, acknowledgements of receipt containing not only the RTT but also cues regarding the buffer memory of the receiver, as described in document JP2001-257715. The sender, through judiciously combined algorithms, would tailor its send stream by considering both types of information.